For the Trigger and Content lines in the Page Layout, hierarchy, I’d suggest adding both placeholder entry in to the Inspector that includes the (?) lookup button. Also add an entry to the Elements Docs for each. If only to link to Modal. While the Modal doc entry does use the word trigger, it doesn’t link to any of the concepts.
With some reading and experience one can piece together the meaning. Though quite new to Elements, I have web dev experience. For new eyes, would save time.